Saturn's Moons and Rings • We have already discussed a few details of Saturn's ring system • It is impossible to fully discuss the rings without mentioning the interactions with Saturn's many moons •_ • The rings themselves may have been formed from a current or past moon Ring Overview • The rings were named alphabetically in order of discovery •_ • The D ring is a faint ring close to Saturn's cloud tops • The F and G rings and faint and narrow, outside the A ring • The E ring is a large, low density ring in a more distant orbit around Saturn What Creates the Rings? • One clue to ring formation is where they are located compared to their host planet • For each of the 4 outer planets, rings are usually not seen anywhere past about 2.4 times the planet's radius •_ 1 The Roche Limit •_ • Remember that gravity acts to squeeze moons that are close to the planet • If the moon comes close enough, the moon is stretched to the point that it is destroyed Ring Formation • The fact that Saturn's rings are inside the Roche limit explains why they do not clump to form a new moon • How did they form in the first place? – They could possibly be left over material from the formation of Saturn –_ – A comet (like Shoemaker-Levy 9) could have come close to Saturn and broken up around the planet – An impact with one of Saturn's current or past moons may have generated the material in the rings How Old Are The Rings? • _ • The rings are very bright, suggesting they haven't been covered by lots of interplanetary dust over time • The interactions and disturbances in the rings suggest they are unstable, and shouldn't last more than a few hundred million years • These points and others lead Disturbance waves in the A ring astronomer to believe the may be no older than 50 million years 2 Ring - Moon Interactions • The spacing of Saturn's rings are based largely on interaction with different moons •_ • In this case, particles in the Cassini Division are in a 2:1 resonance with the moon Mimas • When Mimas and the particles 'sync up' in orbit, Mimas' gravity pulls on the particles and clears the gap Shepherd Moons • Close up images of the F ring showed a very narrow and braided structure • Further analysis has revealed that this ring is controlled by the gravity of two nearby moons, Prometheus and Pandora •_ • Gravitational interactions keeps the F-ring narrow and confined Herding the Rings • The presence of a single moon in the rings acts to spread the particles out and create a gap •_ • These types of interactions change the structure of the ring system over long periods of time 3 The Source of the E-ring •_ • Recent observations of Enceladus have revealed water volcanoes on its surface, which most likely provide the ice particles in the E-ring Saturn's Moons • Like Jupiter, the number of Saturn's moons continues to increase (currently in the 50's) • Unlike Jupiter, Saturn's moons come in all sizes – Small (a few kilometers) – Medium (a few hundred kilometers) – Large (over a thousand kilometers) •_ The Small Moons •_ Janus & Epimetheus • Janus and Epimetheus actually trade orbits every time they pass each other • The small moons Telesto and Calypso share an orbit with the larger Tethys – The two smaller moons lead and trail Tethys by 60o 4 Hyperion • The small moon Hyperion is possibly the weirdest of Saturn's moons • Influences with other moons cause it to 'tumble', never having a constant rotation speed or direction •_ The Medium Moons The Medium Moons • The surfaces of Rhea, Iapetus, Tethys, and Dione resemble the surface of the Moon in many ways •_ • On Dione, there is even some evidence of maria-like terrain meaning there may have one been volcanism of Craters on Rhea some sort 5 Mimas • The moon Mimas is dominated by a huge impact crater named Herschel •_ • The impact probably came close to destroying the moon • The ejecta from this impact may be a possible source for the material in the rings Enceladus • The presence of the E-ring combined with the bright, smoothed surface suggested volcanism on Enceladus •_ • Recent images from the Cassini mission revealed large plumes of water vapor erupting from a hot spot in the crust The Large Moon - Titan • Titan orbits Saturn at a distance of about 1.2 million kilometers •_ • Initial observations of Titan by the Voyager probes revealed a very thick atmosphere • Neither probe was able to glimpse the surface 6 Titan's Atmosphere • The top of Titan's atmosphere is a thin haze that is created by interactions with ultraviolet sunlight •_ • It is made up of mainly nitrogen (>90%), some argon, methane, and traces of many other gases Weather on Titan •_ • This temperature is in just the right range that liquid methane should be abundant • It is believed that methane on Titan acts much like water on Earth in terms of rain, lakes, oceans, and erosion Titan's Surface • The most detailed images of Titan's surface have come only in the past two years • The Cassini-Huygens mission is designed to study Saturn, its rings, and its moons •_ • These images from orbit reveal what may be large areas of liquid on the surface 7 The Huygens Lander • The mission also included a lander which descended through the atmosphere and landed on the surface •_ • Analysis is still being made on all the new data Standing on Titan •_ • The site was strewn with rounded rocks that appear to have been weathered by both wind and liquid • Analysis of the surface material is still taking place An Early Earth? •_ • Many scientists believed the early Earth also contained an atmosphere and surface full of this 'chemical soup', which eventually spawned life • Studying the chemical composition and reactions in Titan's atmosphere may give us insight into Earth's (and life's) early history 8.